The television receiver has become one of the most familiar electronic communication devices throughout much of the world. Equally pervasive are the broadcast and cable systems which provide the television signals upon which these receivers are dependent. While a variety of broadcast and receiver systems are employed, the basic fundamentals of system operation remain common to virtually all television broadcast operations and television receivers. At the broadcast facility, the video or picture information together with the associated sound information is formatted in accordance with the applicable sequential display scan system to be used by the receiver. This is typically accomplished by a television camera within which a local oscillator provides periodic scan signals used by the camera to sequentially scan the image before the camera. Because the proper display of the video or picture information at the receiver requires that the receiver display be, in essence, synchronized to the scan process which was originally used in the camera, the video signal is combined with periodic scan reference signals generally referred to as synchronization signals or simply "sync" signals. The resulting signal is then modulated upon a broadcast channel carrier for distribution either through over-the-air broadcast, cable systems, or recording upon fixed media such as video cassettes.
At the television receiver, the "broadcast" signal is received and processed to recover the video, sound and scan synchronization signals for processing and application to the receiver display. The great majority of television receivers use a cathode ray tube (CRT) as a display device in which one or more electron guns are operative to produce high energy electron beams which energize a light producing phosphor screen and produce a viewable image. Proper image display requires that the electron beams from the electron guns within the cathode ray tube sequentially scan the phosphor screen in both horizontal and vertical directions in close synchronization with the reference sync signals provided within the received signal.
Thus, in a typical cathode ray tube display, the video or picture information is used to modulate the intensity of the electron beams and their resulting illumination of the display screen while the reference scan synchronization signals are used to properly time or sequence the horizontal and vertical direction scan actions of the image-producing electron beams of the electron gun. In monochrome cathode ray tube displays, a single electron gun is directed at a phosphor display screen having a single type of phosphor coating. In most color systems, however, a trio of electron guns corresponding to the primary colors of the systems are directed at a display screen having three different phosphor types arranged in a pattern which facilitates the individual stimulation of each phosphor type by its corresponding color primary electron gun.
The most common type of cathode ray tube scanning system uses electromagnetic yokes supported upon the cathode ray tube which create magnetic fields used to deflect the electron beams produced by the electron guns and accomplish horizontal and vertical scanning. In virtually all commercially available cathode ray tubes, the geometric relationship between the electron guns and the display screen or "faceplate" creates the well known pin cushion distortion and convergence errors. To overcome pin cushion and convergence errors, practitioners have provided pin cushion correction and convergence correction systems which apply additional signal components to the deflection system.
The foundation of the receiver display system is found in the vertical and horizontal scan oscillators and their respective control systems used to regenerate the display scan signals and properly synchronize them to the reference sync signals. In particular, the horizontal oscillator and its synchronizing system form a critical element in the receiver display. The horizontal scan oscillator produces output signals which are used to drive the horizontal deflection yoke of the CRT as well as the high voltage system which provides the required CRT electron beam accelerating potential. The deflection yoke and high voltage system signals are, in turn, used to produce the above-mentioned pin cushion correction and convergence correction signals. In addition, the increasing use of on-screen information displays found in television receivers creates a need for an additional signal component having a frequency which is a multiple of the horizontal scan frequency and which is properly synchronized to the scanning of the cathode ray tube.
Thus, a typical television receiver horizontal scan system requires a number of signals which ideally satisfy a complex set of interrelated criteria. For example, the horizontal scan output device requires oscillator signals precisely synchronized or locked to the incoming sync signals while the systems providing pin cushion and convergence correction require signals synchronized or locked to the actual CRT scan process. In addition, the on-screen display system also requires a horizontal scan frequency multiple signal which is synchronized or locked to the CRT scan.
Because the above systems all utilize various tuned or frequency responsive components, their relationships are largely frequency dependent. While this is not a significant problem in horizontal oscillator and control systems intended for use in a single scan frequency environment, substantial problems are raised if the system is used to satisfy a range of different horizontal scan frequencies. Because the horizontal scan frequencies used in the various television broadcast systems such as NTSC, PAL, HDTV, and nonstandard systems such as computer monitors or the like vary substantially, the maintenance of these relationships between signal components while operating in each of these environments is a difficult and complex task.
Despite these difficulties and complexities, however, there remains a need in the art for a horizontal scan control and clock system which provides the appropriate drive signals for all of the scan related functions and which accommodates a wide spectrum of scan frequencies. Accordingly, it is a general object of the present invention to provide an improved adaptive horizontal scan control. It is a more particular object of the present invention to provide an adaptive horizontal scan control having a horizontal scan locked clock system. It is a still more particular object of the present invention to provide an improved adaptive horizontal scan control and scan locked clock system which accommodates a broad spectrum of horizontal scan frequencies.